Cambridge: Helminth Infections of Man

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http://www.path.cam.ac.uk/~schisto/General_Parasitology/Hm.helminths.html

 

 

 

Cambridge

 

 

 

General Parasitology Pages

 

Helminth Infections of Man

 

 

 

This page aims to introduce the student to the diversity of helminth

infections in man, and even more importantly, to the numbers of

individuals that harbour these infections in all regions of the world.

There are three major groups of helminths containing members that have

man as their main hosts, these being the Digenean Flukes, the

Tapeworms (Cestodes) , and the Roundworms (Nematodes). These are dealt

with in greater detail on specific pages in this web site (see the Web

Site Guide or navigate to the Digenean, Cestode and Nematode Pages

using the links in the side margin, or for specific parasites use the

links within the text).

 

Nematode infections

 

Enterobius vermicularis - Pinworm, Threadworm.

 

An extremely common nematode infection, particularly in

temperate areas such as Western Europe and North America, (it being

relatively rare in the tropics) and particularly in children. It has

been estimated that the annual incidence of infection is over 200

million, this probably being a conservative figure. Samples of

caucasian children in the U. S. A. and Canada have shown incidences of

infection of fro 30% to 80%, with similar levels in Europe.

 

 

 

 

Ascaris lumbricoides - The Large Human Roundworm.

 

Again the incidence rates for this parasite are very high with

> 1500 million cases of infection annualy, of which ~210 million of

these cases are symptomatic (* but see below).

 

 

 

 

Trichuris trichuria - The Large Human Roundworm.

 

The incidence rates for this parasite are also very high, with

estimates of ~1300 million cases of infection annualy, of which >133

million of these cases are symptomatic (* again but see below).

 

 

 

 

The Hookworms .

 

These are represented by two parasites, Necator americanus in

the tropics and sub tropics worlwide and the S. E. states of the U. S.

A., and Ancylostoma duodenale, again with a worldwide distribution in

the tropics and sub tropics as well as the Mediterranean region. In

the case of these parasites there are > 1200 million cases of hookworm

infection annualy, of which ~100 million of these cases are

symptomatic (* again, but see below).

 

 

 

 

Lymphatic filariasis - Elephatiasis

 

This disease is caused principaly by two parasites, Wuchereria

bancrofti with an annual rate of infection of ~106 million cases, and

Brugia malayi with an annual rate of infection of ~12.5 million. The

total number of people infected with other types of lymphatic filarial

worms is much smaller, at ~1.5 million cases. These lymphatic filarial

worms, (along with the related filarial parasite Onchocerca volvulus,

are unusual among the nematodes in that they deveope with, and are

transmitted by insect vector intermediate hosts.

 

 

 

 

Onchocerca volvulus - River Blindness.

 

The incidence rates for this parasite are not as high as some

of the previously described parasites, with an annual rate of

infection of ~18 million, but due to the extreme pathology associated

with this parasite, often with all adult members of affected villages

losing their sight, along with severe skin conditions.

 

 

 

 

Dracanculus medinensis - Guinea Worm.

 

The incidence rates for this parasite are much lower, with an

estimated annual rate of infection of ~100 000. This is much lower

than in the recent past, when up to 50 million people were infected,

and this reduction illustrates how successfull helminth control

programs can be effective in reducing the disease caused by these

organisms.

 

 

 

Other important nematode infections include Trichinella spiralis,

Strongyloides stercoralis, and a number of more rare infections.

Nematodes that normally infect other animals may still cause disease

in man. These include Toxocara canis and a number of nematodes causing

Anisakiasis.

 

Digenean Trematode Infections

 

Schistosomiasis - Bilharzia.

 

This disease is the focus of this parasitology web site, and

in terms of morbidity and mortality is the most important human

helminthiasis. The numbers infected are lower than those of many of

the nematode infections, with an estimated annual incidence of

infection of > 200 million cases. In terms of active disease however

the parasite is much more important, with an estimated annual

mortality rate of ~1 million deaths directly due to infection with

these parasites.

 

 

 

 

Opisthorchis sinensis - The Chinese Liver Fluke

 

This is also a very important trematode infection, with an

estimated annual incidence of infection of ~20 - 30 million cases,

mostly in the Far East, in Japan, China, Taiwan and South East Asia.

 

 

 

 

Paragonimus sp. - The Lung Fluke

 

This fluke causes a pulmonary disease, the adult parasites

living in the lungs of their definitive hosts (e.g. man). There are a

number of different species of this parasite, the most well documented

being P. westermani in the Far East. Here it may be locally very

common, with up to 40 to 50% of the population infected.

 

 

 

There are a number of other digenean trematode infections. These

include various Echinostome infections as well as a number of other

flukes, described in a seperate Human fluke page within this site. In

addition there are a number of these parasites that usually infect

domesticated animals, but are also cause well known human infections

as well. These include Fasciola hepatica and Dicrocoelium dendriticum.

 

Cestode (Tapeworm) Infections

 

Taenia saginata - The Beef Tapeworm

 

This only causes very limited pathology in man, but the anual

incidence of infection is high, at an estimated 50 million cases.

 

 

 

 

Taenia solium - The Pork Tapeworm

 

This has a similar estimated annual incidence of infection of

~50 million cases. However in this case the consequences may be more

severe, due to the added risk of contracting infection with the larval

metacestode, (cysticercosis). This may have extreme consequences in

terms of the pathology associated with infection, with an estimated

annual mortality rate of ~50 000 deaths.

 

 

 

For the Cestodes these annual incidence rates are based on detection

of infection with the adult parasite. This is achieved by examination

of faeces, urine or sputum for parasite eggs . Diagnosis of infection

with larval metacestode parasites, such as Echinococcus sp. is very

difficult, due to the lack of non invasive diagnostic techniques. It

is in consequence very difficult to estimate annual rates of

infection, even though these metacestodes may be very important pathogens.

 

 

links

An Introduction to Cestode Biology

 

A Introduction to the Digeneans

 

A Introduction to Nematode Biology

 

General Parasitology

 

The Taxonomy of the Parasitic Helminths

 

 

 

Last Modified on 5th October 1998